Fair n-way interleaving: given a finite number of (possibly infinite)
lists, produce a single list such that whenever v has finite index in one
of the input lists, v also has finite index in the output list. No list's
elements occur more frequently (on average) than another's.

Unfair n-way interleaving: given a possibly infinite number of (possibly
infinite) lists, produce a single list such that whenever v has finite
index in an input list at finite index, v also has finite index in the
output list. Elements from lists at lower index occur more frequently, but
not exponentially so.

Slightly unfair 2-way Cartesian product: given two (possibly infinite)
lists, produce a single list such that whenever v and w have finite
indices in the input lists, (v,w) has finite index in the output list.
Lower indices occur as the fst part of the tuple more frequently, but not
exponentially so.

Slightly unfair n-way Cartesian product: given a finite number of
(possibly infinite) lists, produce a single list such that whenever vi has
finite index in list i for each i, [v1, ..., vn] has finite index in the
output list.

Very unfair 2-way Cartesian product: same guarantee as the slightly unfair
one, except that lower indices may occur as the fst part of the tuple
exponentially more frequently. This mainly exists as a specification to test
against.

Very unfair n-way Cartesian product: same guarantee as the slightly unfair
one, but not as good in the same sense that the very unfair 2-way product is
worse than the slightly unfair 2-way product. Mainly for testing purposes.